Roofs used to cover electric arc furnaces so as to prevent heat being dispersed from inside the furnace, and to prevent the leakage of noxious fumes and waste, are known to the state of the art.
These roofs normally have a substantially central aperture to insert, position and move the electrodes and a peripheral aperture, called the fourth hole, used in cooperation with intake and discharge conduits in order to take in the fumes and volatile waste from inside the furnace and carry them to the processing and purifying means and thence to the stack.
Given the working conditions inside the furnace, and in particular the extremely high temperatures which develop inside the furnace, there is a known need to provide systems to cool the roof, normally in cooperation with the inner surface of the roof.
This cooling is usually carried out by means of tubes or conduits structured as panels wherein the cooling fluid circulates.
One example of such cooling panels is described in EP-A-0 140 401.
The function of these cooling panels is to prevent the roof from over-heating and therefore to protect it from wear and from damage, and thus extend its working life.
A problem which has to be faced when these cooling devices known to the state of the art are installed is the lack of homogeneity in the distribution of temperatures on the inner surface of the roof.
In fact it is well known that, during the operating cycle of the furnace, the temperature is much higher in the central part of the roof, near the electrodes, than at the periphery.
Moreover, the temperature of the roof near the outlet opening, or fourth hole, is much higher than the temperature developed at the opposite side, and increases progressively as it approaches the fourth hole because of the considerable flow of incandescent fumes towards this area.
The intake systems connected with this fourth hole also determine a concentrated intake on a limited part of the whole furnace, and consequently cause localized wear and damage.
Systems to cool the roof which are known to the state of the art are not always able to guarantee the optimum heat insulation and protection which can prevent localized wear in those parts of the furnace which are most subject to over-heating.
Moreover these known devices give a heat exchange coefficient, or removal of the heat flow, which is substantially uniform over the whole surface of the roof, with the result that over all the roof it is necessary to guarantee a heat exchange coefficient at least equal to that required in the hottest part of the furnace, that is to say, near the fourth hole.
Consequently, for a large part of the inner surface of the roof the cooling system is out of proportion, thus causing a great consumption of energy and an excessive quantity of cooling fluid being used, whereas the hottest areas always work at a very high temperature, with the risk of break-downs and breakages in the cooling conduits.
State of the art conduits may be circular, conformed as a ring or as a spiral, or they may be radial from the centre of the roof towards the periphery or vice versa.
However, these conduits, even when they are structured as panels, in most cases are arranged substantially on a single horizontal plane cooperating with the inner part of the furnace. This solution does not allow, except to a very limited degree, insulating material such as waste to accumulate; and yet the accumulation of waste or other material could greatly assist the panels in their action of cooling and heat insulation.
Moreover, all those cooling systems described exercise a cooling action which is substantially uniform over all the surface of the roof, given the constant flow of cooling water circulating in the conduits.
The state of the art also covers jet-type cooling devices, which use jets of water cooperating with the outer surface of the roof, where the water is sprayed and runs on the outer surface and is collected in the peripheral area.
In this case it is possible to distribute the jets of water in such a way as to obtain a greater cooling in the hottest points, but then there is the problem that a greater flow of water is obtained in the outer peripheral area, where a lesser removal of heat is required.
A further problem which affects the working life of roofs cooled according to systems known to the state of the art, is that there are welds between the single elements of the cooling conduits.
These welds form critical points and create tensions along the conduit which cannot be completely eliminated even by such heat treatments as tempering.
These tensions, together with the particular conditions of high temperature to which the pipes are subjected, may cause the welds to break, with the resulting leakage of cooling water into the furnace.
Given the high pressure of the water circulating in the cooling conduits, the amount of water which in this case penetrates the furnace is very high, and as soon as it comes into contact with the molten metal it evaporates very quickly, with a consequent sudden rise in pressure which may cause an explosion.
Such a situation requires that the furnace be closed down immediately, with all the technical and economic problems that this entails, apart from the potential danger for the workers.